This invention relates to in situ recovery of shale oil and, more particularly, to techniques for attenuating seismic shock produced when detonating large mounts of explosive for forming an in situ oil shale retort.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598 which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supply hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting." Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooled oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred herein as "liquid and gaseous products."
U.S. Pat. No. 4,043,595 discloses a method for explosively expanding formation containing oil shale to form an in situ oil shale retort. According to a method disclosed in that patent, formation within a retort site is excavated to form a columnar void bounded by unfragmented formation having a vertically extending free face. Blasting holes are drilled adjacent the columnar void and parallel to the free face. In one embodiment the columnar void is cylindrical and the blasting holes are arranged in concentric rings around the columnar void. In another embodiment, the columnar void is a slot having large parallel, planar vertical free faces toward which the formation in the retort site can be explosively expanded. The blasting holes are arranged in planes parallel to such free faces. Explosive is loaded in the blasting holes and detonated in a single round. This produces a large explosion which explosively expands the formation adjacent the columnar void toward the free face to form a fragmented permeable mass of formation particles containing oil shale which occupy the columnar void and the space in the retort site occupied by unfragmented formation prior to such explosive expansion.
Explosive in such blasting holes is detonated in a time-delay sequence so that unfragmented formation within the retort site is explosively expanded in segments progressing away from the free face provided by the columnar void. The sequence of blasting is rapid, and in an embodiment disclosed in U.S. Pat. No. 4,043,595, the time-delays for explosively expanding formation toward the columnar void span a time period of less than 700 ms. Shorter time-delays can be used in other embodiments. In one embodiment, as much as 85 tons of explosive are detonated in a single round for explosively expanding formation toward a columnar void. This produces a powerful explosion which generates seismic shock waves traveling outwardly through unfragmented formation extending away from the blasting site. Seismic shock from such a powerful explosion can cause serious damage to equipment and structures located in underground workings near the blast site. Equipment which can be damaged from such seismic shock cannot necessarily be easily or economically removed from underground workings prior to such explosive expansion. Thus, there is a need to attenuate the seismic effect on sensitive equipment in underground workings caused by detonating large amounts of explosive for forming an in situ oil shale retort.